1. Field of the Invention
The present invention relates to a semiconductor device which is used for mobile equipment, such as cellular phones, and so on, and which is intended to exhibit low power consumption through a power cutoff technique.
2. Description of the Related Art
In recent years, power consumption during standby of mobile equipment, such as cellular phones, etc., has become a problem. Thus, in order to reduce the power consumption during standby, a technique is used which interrupts voltage supply (cuts off power supply) to semiconductor devices to reduce off-leakage current at transistors. By recent finer design rules in semiconductor process, the threshold value of transistors is also decreased to induce more off-leakage at transistors, which makes power cutoff technique very useful. The power cutoff technique of reducing off-leakage current to implement low power consumption is in the actual use (see, for example, Japanese Patent Laid-Open No. 269854/1997). In conventional power cutoff, leakage current can be completely suppressed by externally stopping voltage application to source lines. In recent years, however, a voltage to drive transistors is lowered due to the resistive components during circuit operation, which degrades circuit performance. In order to suppress the circuit performance degradation, a technique is adopted which prevent a voltage drop by giving much parasitic capacity to power supply.
As described above, however, in the technique of giving much parasitic capacity to power supply in order to suppress the circuit performance degradation, the power supply capacity requires much charging time at the time of system recovery after power cutoff and it takes much time to perform the rise of a system. Also, due to the long charging time to meet the power supply capacity, the rise of power supply to drive transistors varies by transistor. As a result, flow-through current, which flows through the transistors, arises to consume electric power.